There is a strong demand from automotive manufacturers and other users for a plated steel sheet having an excellent corrosion resistance even in a severe corrosive environment and capable of improving durability of the product. As a galvanized steel sheet capable of coping with such a demand, the iron-zinc alloy galvanized steel sheet having an iron-zinc alloy coating formed on at least one surface of a steel sheet is known. The iron-zinc alloy galvanized steel sheet has many practical advantages such as an excellent corrosion resistance after painting, a low manufacturing cost and easy reuse of scrap thereof as an iron source after melting.
The following processes for manufacturing such an iron-zinc alloy galvanized steel sheet are known:
(1) Alloying process:
By rapidly heating and then rapidly cooling a zinc-galvanized steel sheet having a zinc coating formed on at least one surface of a steel sheet, the zinc coating is converted into an iron-zinc alloy coating.
(2) Electro-galvanizing process:
By subjecting a steel sheet to an electro-galvanizing treatment in a galvanizing bath mainly comprising zinc ions and iron ions, an iron-zinc alloy coating is formed on at least one surface of the steel sheet.
However, the iron-zinc alloy galvanized steel sheet manufactured by the alloying process described under (1) above is hardened since the iron-zinc alloy galvanized steel sheet is rapidly heated and then rapidly cooled to convert the zinc coating into an iron-zinc alloy coating. Therefore, the iron-zinc alloy galvanized steel sheet manufactured by the alloying process has a problem in that cracks and wrinkles are caused in the steel sheet when applying a severe forming such as that by a press.
The iron-zinc alloy electro-galvanized steel sheet manufactured by the electro-galvanizing process described under (2) above, in contrast, is not hardened since it is not applied with rapid heating and rapid cooling as the iron-zinc alloy galvanized steel sheet manufactured by the alloying process. Therefore, the iron-zinc alloy electro-galvanized steel sheet manufactured by the electro-galvanizing process does not pose the problem of occurrence of cracks or wrinkles in the steel sheet even by applying a severe forming such as that by a press. The iron-zinc alloy electro-galvanized steel sheet manufactured by the electro-galvanizing process is suitable for use as the outer sheet of car because of the excellent formability.
However, the iron-zinc alloy galvanized steel sheet manufactured by the alloying process and the iron-zinc alloy electro-galvanized steel sheet manufactured by the electro-galvanizing process have a drawback of being susceptible to powdery peeloff (hereinafter referred to as "powdering") of the iron-zinc alloy coating when subjected to a severe forming such as that by a press. However, almost no measure has so far been taken against occurrence of such powdering.
On the other hand, a cationic type electro-depositing process is popularly applied by automotive and other manufacturers to form a paint film on the surface of an iron-zinc alloy galvanized steel sheet. However, when forming a paint film on the surface of an iron-zinc alloy galvanized steel sheet by the cationic type electro-depositing process, hydrogen gas produced during electro-deposition and entrapped in the paint film causes generation of crater-like pinholes in the paint film. Since these crater-like pinholes produced in the paint film form a defect in appearance of the paint film surface, it is necessary to prevent occurrence thereof. Crater-like pinholes tend to occur when the ratio of zinc content to the coating weight is high. Occurrence of pinholes can therefore be prevented by reducing the ratio of zinc content to the coating weight, whereas a lower ratio of zinc content to the coating weight leads to deterioration of bare corrosion resistance of the iron-zinc alloy coating itself.
As an iron-zinc alloy galvanized steel sheet serving to solve the above-mentioned problems, an iron-zinc alloy galvanized steel sheet for cationic electro-deposition of paint having a plurality of iron-zinc alloy coatings is disclosed in Japanese Patent Publication No. 58-15,554 dated Mar. 26, 1983, which comprises:
at least one iron-zinc alloy coating as the lower layer, formed on at least one surface of a steel sheet, the ratio of zinc content to the coating weight of each of said at least one iron-zinc alloy coating as the lower layer being over 40 wt.%; and
an iron-zinc alloy coating as the upper layer, formed on said at least one iron-zinc alloy coating as the lower layer, the ratio of zinc content to the coating weight of said iron-zinc alloy coating as the upper layer being up to 40 wt.% (hereinafter referred to as the "prior art 1").
The above-mentioned prior art 1 involves the following problems:
(1) When applying a severe forming such as that by a press, powdering may be caused in the at least one iron-zinc alloy coating as the lower layer and the iron-zinc alloy coating as the upper layer.
(2) A considerable difference in the ratio of zinc content to the coating weight between the uppermost coating of the at least one iron-zinc alloy coating as the lower layer and the iron-zinc alloy coating as the upper layer leads to a large difference in the electrode potential between the uppermost coating of the at least one iron-zinc alloy coating as the lower layer and the iron-zinc alloy coating as the upper layer, and this causes galvanic corrosion in the iron-zinc alloy coatings, thus deteriorating bare corrosion resistance of the iron-zinc alloy coating itself and corrosion resistance after painting.
On the other hand, as an iron-zinc alloy electro-galvanized steel sheet excellent in bare corrosion resistance of the iron-zinc alloy coating itself and corrosion resistance after painting, an iron-zinc alloy electro-galvanized steel sheet having a plurality of iron-zinc alloy coatings is disclosed in Japanese Patent Provisional Publication No. 58-67,886 dated Apr. 22, 1983, which comprises:
a plurality of iron-zinc alloy coatings formed on at least one surface of a steel sheet, the ratio of zinc content to the coating weight of each of said plurality of iron-zinc alloy coatings gradually decreasing or increasing, within the range of from 5 to 98 wt.%, from the innermost coating toward the outermost coating of said plurality of iron-zinc alloy coatings, and the total coating weight of said plurality of iron-zinc alloy coatings being within the range of from 5 to 50 g/m.sup.2 per side of said steel sheet (hereinafter referred to as the "prior art 2").
The above-mentioned prior art 2 involves, as in the prior art 1, the following problems:
(1) When applying a severe forming such as that by a press, powdering may be caused in the plurality of iron-zinc alloy coatings.
(2) A considerable difference in the ratio of zinc content to the coating weight between two adjacent coatings among the plurality of iron-zinc alloy coatings leads to a large difference in the electrode potential between the above-mentioned two adjacent coatings, and this causes galvanic corrosion in the iron-zinc alloy coatings, thus deteriorating bare corrosion resistance of the iron-zinc alloy coating itself and corrosion resistance after painting.
Under such circumstances, there is a strong demand for the development of an iron-zinc alloy electro-galvanized steel sheet excellent in bare corrosion resistance of the iron-zinc alloy coating itself and corrosion resistance after painting, which is not susceptible to powdering in the iron-zinc alloy coating even through a severe forming such as that by a press, but an iron-zinc alloy electro-galvanized steel sheet provided with such characteristics has not as yet been proposed.